The invention relates to a complementary shutdown device for an undermoderated nuclear reactor.
A reactor of this type comprises a core consisting of fuel arrays of two different types, arranged side by side, some of them, referred to as fissile arrays, mainly containing a fissile material, whereas the others, referred to as fertile arrays, contain a material which is capable of producing fissile material under the action of neutron bombardment. The fertile arrays are generally arranged at the periphery of the core, where they collect the neutron radiation produced by the fissile arrays.
The reactor core is immersed in a moderating fluid, such as water, which generally also serves as a heat-transfer fluid. This fluid circulates in contact with the fuel elements arranged inside the arrays.
Compared with pressurized water reactors of the conventional type, which only comprise fissile arrays, undermoderated reactors have a very much lower ratio volume of moderator to volume of fissile material in the core, or moderating ratio VM/VU, whereby the energy spectrum of the neutrons is very different.
This spectrum is intermediate between that of the conventional pressurized water reactors, in which the neutrons are very slow, and that of fast reactors, in which the neutrons are not slowed down. This spectrum is said to be epithermal. A spectrum of this type makes it possible to produce fissile material from fertile material arranged, for example, at the periphery of the core.
In undermoderated reactors, a lower moderating ratio is obtained by making provision for a distribution of the fuel material in the fissile arrays which assures optimum contact with the moderator and at the same time a low volume ratio of the moderator to the fissile material.
Whether fissile or fertile, the fuel arrays consist of a prism-shaped casing made of a weakly neutron-absorbing material, such as a zirconium alloy, inside which long tubular rods, containing the fissile material or the fertile material, are arranged parallel to the height of the casing and in a uniform lattice in the cross-sections of this casing.
The spacing between the rods, inside the array, is generally maintained by spacer devices, such as wires wound in spirals around the can of the rods. This arrangement enables the heat-transfer and moderating fluid to circulate in contact with the whole surface of the rod, while permitting a very small spacing between the rods, i.e., it makes it possible to obtain a very dense lattice in which, however, the cans of the rods are never contiguous. This high-density lattice of rods of fissile material makes it possible to obtain a low moderating factor.
Inside the arrays, certain positions in the bundle are reserved for guide tubes which pass through the array over almost the whole of its height and which make it possible to introduce, into the latter, the rods of strongly neutron-absorbing material, joined together in the form of clusters forming the control rods of the reactor.
The controlled displacement of these control rods, so as to introduce the rods of absorbing material to a greater or lesser extent into the guide tubes of certain arrays, makes it possible to adjust the reactivity of the reactor core.
In the event of an emergency shutdown of the reactor, all the control rods are allowed to drop into the reactor core in the position of maximum insertion, which introduces a maximum anti-reactivity of the order of 7,000 pcm.
In conventional pressurized water reactors, which are not used to convert fertile material to fissile material, a second system makes it possible to render the core subcritical in the event of a malfunction. This is the boric acid emergency system. In fact, in these reactors, the operation of the reactor is assured simultaneously by the control rods and by the introduction of boron, in the form of boric acid, into the reactor cooling water.
In undermoderated reactors, where it is desired to have a low moderating ratio, no system for the introduction of boric acid into the cooling water is really effective.
In the event of failure of some of the control rods, there are therefore no means of rendering the core subcritical, in spite of everything, in all cases.
In fast fission nuclear reactors, a complementary shutdown device for the reactor is known, consisting of articulated absorbing elements which can be introduced into certain arrays in the event of a malfunction of the main control system of the reactor.
However, a device of this type cannot be used in the case of undermoderated reactors, the structure of which is virtually identical to that of conventional pressurized water reactors.